1. Field of the Invention
The present invention relates to detection and more particularly to a detection network which is convertible for operation on either an amplitude modulated signal or on a frequency modulated signal. The invention also relates to a detection circuit suitable for integrated circuit fabrication.
2. Description of the Prior Art
A known FM demodulator obtains a quantity containing the modulation information by employing a pair of tuned circuits to form a pair of quadraturely related signal components and by employing a pair of diodes to rectify the resultants, derived by combining these phase components. In this known FM demodulator, one signal component is of reference phase and another signal component has a phase which is a linear function of the frequency deviation, being in quadrature at resonance. These two components are combined to form a first resultant, which is applied to one diode and combined to form a second resultant which is applied to the other diode. Thus, a pair of resultant vectors are created whose inequality reflects the frequency deviation. After rectification by the diodes, the d.c. components of the resultant vectors may be substracted to obtain an output quantity replicating the modulation information.
Another known FM demodulator employs two quadrature waves and gating to produce variable width output pulses which are later integrated. In this demodulator, gating is performed by a gated beam tube having a pair of control grids, each capable of cutting off conduction. The in-phase signal is applied to a first control grid and a quadrature signal is developed on a second control grid the quatrature component being self-generated in a auxiliary resonant circuit. Thus, depending upon whether the input signal is above or below the resonant frequency of the auxiliary resonant circuit, the phase relationship will vary about quadrature and cause a variation in the period when both grids permit conduction. Assuming a conduction angle of 90.degree. at resonance, the conduction period is arranged to fall as the frequency increases above resonance and to rise as the frequency falls below resonance. Thus, a sequence of variable width pulses is produced, whose width is proportional to the frequency deviation of the signal. If these pulses are integrated at an audio rate, the modulation information may be recovered. While sound in principle, this vacuum tube demodulator is uneconomical and obsolete for most applications today.
More recently, FM demodulators in integrated circuit form have employed two-quadrant and four-quadrant multiplier circuits in which the quadrature related components are applied respectively to the upper and lower ranks of the multipliers. These multipliers often exhibit substantial phase shifts due to unavoidable parasitics between signals coupled to the lower and to the upper ranks. These phase shifts also change as a function of the signal level. Thus, if the device is optimized for normal signal levels, one obtains relatively poor low signal performance. A disadvantage particularly of the two quadrant multiplier, is in the d.c. offset which may be added to the demodulated signal and which may affect the interchangeability of the resulting integrated circuits. In the two quadrant multiplier, the d.c. voltage offset at the input junctions is multiplied by the d.c. gain, and appears as a larger uncertainty in the d.c. output level.